The present invention relates to a method of treating copper foil for use in the production of printed circuit boards to improve the foil's adhesion to a base material, and also to laminates formed as a result of said treatment.
Copper foil used in the manufacture of printed circuit boards is usually bonded to a dielectric base material and many proposals have been made heretofore for improving the strength of the bond between the copper foil and the base material, whilst at the same time attempting to reduce or eliminate the undesirable characteristics commonly known as `brown staining` and `undercut` which are revealed after etching.
It has become well-established practice to provide a nodular or dendritic layer of copper on the matt side of the copper foil, said layer being encapsulated by an electroplated layer of smooth copper to provide an adhered structure which offers physical traps for the epoxy resin of the base material during lamination thereto. Prior to lamination, the treatment is passivated, for example by chromate dipping or by cathodic electrolysis of a chromate solution.
However, it is acknowledged that such laminates can suffer from, amongst other things, the aforementioned `brown staining` which is believed to be as a result of chemical reaction between the copper of the encapsulating layer and certain components of the epoxy resin mix. This defect can result in a lowering of the insulating properties of the base material as well as a lowering of the peel strength of the copper foil to said base material.
In order to overcome this problem, barrier layers containing zinc, typically of zinc or brass, have been applied over the encapsulating copper in an attempt to separate the copper from the epoxy resin, said barrier layers being passivated in the usual manner.
The provision of pure zinc barrier layers reduced the degree of `brown staining` quite significantly, but other deficiencies remained. More particularly, it was found that, during lamination to the base material, considerable alloying of the zinc of the barrier layer with the underlying encapsulating copper layer occurred, illustrated by the brass colour the treated foil developed. In many instances this caused a substantial loss of peel strength.
Increasing the thickness of the zinc barrier layer can reduce this effect with consequential improvement in peel strength. However the product then becomes prone to the `undercutting` phenomenon during etching, caused by attack of the etchant on the unconverted zinc layer, whereby the bond of the copper track to the insulating board is significantly undermined.
The provision of brass barrier layers obviated the above-mentioned problems associated with pure zinc, but the `anti-staining` characteristics of electroplated brass were not perfect, and, with certain chemically-reactive epoxy layers, an unacceptable level of `brown staining` remained.
In our co-pending U.S. patent application Ser. No. 425,081 we disclose a treatment which includes the step of providing a barrier layer between an encapsulating layer of copper and the epoxy resin, which barrier layer comprises an alloy of zinc and nickel. Such barrier layers, within certain chemical composition limits, combine the necessary inertness with ease of etching and absence of undercutting, especially when passivated.
However, in certain circumstances there is a tendency for the nickel zinc alloy to exhibit a higher degree of powderyness than is desirable, which can result in the transfer of microcrystalline particles of the alloy onto the laminate.